Unexamined Japanese Patent Publication No. Hei-09-237938 has disclosed that a (111) face of metal nitride having a sodium chloride structure as an undercoat layer is used as a substrate for obtaining Group III nitride compound semiconductor layers of good crystallinity. That is, in this official gazette, metal nitride having a sodium chloride structure is used as a substrate and Group III nitride compound semiconductor layers are grown on the (111) face of the metal nitride.
Unexamined Japanese Patent Publication No. Hei-10-321954 has also disclosed that an electrically conductive layer of transition-metal nitride and having a sodium chloride or hexagonal structure is interposed between a substrate and a Group III nitride compound semiconductor layer. As examples of the transition-metal nitride, TiN, VN, ZrN, NbN, HfN and TaN have been shown.
The present inventors have made eager examination while paying attention to the metal nitride, particularly TiN. As a result, it has been found that there is fear that difference in lattice constant between TiN and a Group III nitride compound semiconductor may have influence on the crystallinity of the Group III nitride compound semiconductor. Incidentally, the lattice constant of TiN with a face c is 0.299 nm whereas the lattice constant of GaN as a Group III nitride compound semiconductor is 0.319 nm.
On the other hand, characteristic (such as stiffness, impact resistance, etc.) for sustaining the function of a semiconductor device is required of a substrate for the semiconductor device. When the substrate is made of metal nitride, it is conceived that the substrate needs to have a thickness of 50 μm or larger in order to sustain the characteristic. Metal nitride having such a thickness has been never provided as a raw material of an industrial product used for producing a semiconductor.
An object of the invention is to bring the lattice constant of metal nitride, which nitride serves as an undercoat layer under a Group III nitride compound semiconductor layer, closer to the lattice constant of the Group III nitride compound semiconductor to thereby improve the crystallinity of the Group III nitride compound semiconductor layer.
Another object of the invention is to make it possible to form a Group III nitride compound semiconductor layer of a good crystallographic structure by using an industrially easily available raw material. Hence, a semiconductor device according to the invention has a semiconductor layer of a good crystallographic structure and can be produced inexpensively.
A further object of the invention is to provide a Group III nitride compound semiconductor device of a novel configuration and a method for producing the same.